The present invention relates to the synthesis, characterization, and use of intermetallic magnetic iron-palladium nanoparticles, particularly as magnetically recoverable catalysts.
In its classical definition, a catalyst is a composition (typically an element or compound) that increases the rate of an underlying chemical reaction without participating as a reactant or a product in the reaction and thus without being used up as the reaction proceeds.
Although this definition is accurate and appropriate on a theoretical and molecular level, when used in larger (e.g., “commercial” or “scale up”) amounts, catalysts can present practical difficulties along with their rate-enhancing advantages.
Many catalysts are described as being either homogeneous or heterogeneous. A homogeneous catalyst operates in the same phase as the reaction that it catalyzes. For example, in a reaction that takes place in solution, a homogeneous catalyst is also soluble in that solution. Homogeneous catalysts have advantages, particularly in terms of selectivity, but because they operate in the same phase as the reaction, they are more prone to degradation and they are almost impossible to recover and reuse.
In this regard, a catalyst is considered advantageous if it can be recovered and reused multiple times without significant difficulty and while maintaining a high degree of catalytic performance. Such characteristics are sometimes quantified using the “turnover number” which represents the number of times a catalyst can be used while maintaining a specified level of catalytic activity, often measured by product yield.
Heterogeneous catalysts are present in a different phase from the reactions that they catalyze. As a result, they can be somewhat easier to recover, but because of the phase difference, they can be somewhat less selective than similar homogeneous catalysts and can be sterically hindered in some circumstances. Additionally, even though heterogeneous catalysts can be somewhat easier to recover than homogeneous catalysts, they still require physical recovery steps such as filtration or centrifuging. Furthermore, because a heterogeneous catalyst typically includes the catalyzing element (often a metal) on a physical support (carbon being exemplary), the catalyst can become dissociated from the support during the reaction and thus can be difficult to remove from the final product. This is sometimes referred to as the catalyst leaching.
As one example, when palladium catalysts (e.g., palladium catalysts on carbon supports) are used in the synthesis of pharmaceutical products in scale up (i.e., commercially viable) amounts, palladium metal has a tendency to leach from the support. When separated from the support, the palladium can remain behind as an undesired contaminant in the final product when the support is removed. The presence of biologically-active amounts of heavy metals such as palladium is, of course, usually unacceptable in a pharmaceutical product.
Thus, when the final product of the catalyzed reaction is, for example, a pharmaceutical composition and the catalyst is a heavy metal, the presence of the leftover heavy metal catalyst must be either eliminated or reduced to acceptable amounts, which typically are in the parts per million (ppm) range or less.
As an additional problem, the carbon used to support the palladium catalyst also has a tendency to absorb undesired compositions as the underlying reaction proceeds. This in turn can make the catalyst support unacceptable for future use and can create another disposal problem.
Palladium is nevertheless a preferred catalyst for a number of organic reactions including reactions that are important in the synthesis of higher complexity organic molecules. Examples include (but are not limited to) the Suzuki, Heck, and Sonogashira reactions.
The Suzuki reaction is a coupling reaction between an aryl halide and an aryl boronic acid catalyzed by palladium metal. The Heck reaction is the chemical reaction of an unsaturated halide with an alkene and a strong base using a palladium catalyst to form a substituted alkene. The Sonogashira reaction is a coupling of terminal alkynes with aryl or vinyl halides for which palladium and copper are the catalysts.
Accordingly, a need exists for heterogeneous catalysts that are recoverable, reusable (the terms have slightly different meanings in this art), active, can be readily synthesized, and that are appropriate for scale-up synthesis.
In many cases, magnetic particles offer advantages for heterogeneous catalyst support (among other uses) because they can be easily separated using an external magnetic field. This provides an easier work up procedure that tends to recover all of the solid catalyst in the separation procedure.
Such particles, including nanometer-scale magnetic, solid supported palladium catalysts, are typically formed by one of three different methods and form three somewhat different types of compositions. In a number of cases, an iron composition—often an iron oxide—provides the desired magnetic characteristics.
In the first method, referred to as “deposition/impregnation”, the catalyst (e.g., palladium) is produced by a technique that places the palladium compound on the desired support. In such compositions the palladium is physically attached to the carrier (e.g., iron oxide) rather than chemically bonded to it. As a result, when impregnated catalysts are in use, palladium tends to constantly dissociate from the support and leach into the products, leading in turn to the need to remove the palladium from the desired product.
In the second method, the palladium is covalently bonded, often through organic ligands to an iron oxide support. Although the covalent bond is typically stronger than the physical attraction in the impregnated catalysts, the covalent bond will tend to dissociate under reaction conditions and produce undesired palladium in the reaction product.
Impregnated or covalently bonded magnetic solid supported palladium catalysts are relatively easy to prepare, recover and reuse. As disadvantages, however, the preparation steps usually involve coating or functionalizing steps and are often time-consuming.
In the third method an intermetallic compound is formed that includes the desired catalyst metal. Because of the metal-metal bond, such intermetallic compounds minimize or eliminate leaching. This also makes such catalysts more readily reusable.
As potential disadvantages, however, typical routes for producing intermetallic iron palladium nanoparticles require the use of iron carbonyl (Fe(CO)5) which is extremely toxic and sensitive to exposure to air, characteristics that make iron carbonyl dangerous and inconvenient to work with. Many intermetallic iron-palladium synthesis routes also require higher temperatures in a toxic high boiling point solvent, or annealing at high temperatures (above 500° C.) for up to 15 hours in a mixture of argon and hydrogen gases, or both.
Accordingly, a less toxic (“greener”) approach to obtaining magnetic palladium-iron nanoparticles would provide advantages during synthesis of the nanoparticles as well as during their use in catalyzing reactions and in recovering the catalyst from the reaction products easily and while minimizing or eliminating the problems caused by palladium bleaching or carbon absorption.